In recent years, various techniques of communication between a vehicle unit and a portable unit are used as a key-less entry system, a smart entry system and the like, and these systems use a radio signal transmitted from the portable unit and received by the vehicle unit for authenticating a unique digital data extracted from the radio signal to, for example, open a door of the vehicle by activating an actuator. (Refer to, for example, Japanese Patent Documents JP-A-H09-41754, JP-A-2000-170420, JP-A-2001-98810) (Contents of JP-A-H09-41754 and JP-A-2001-98810 are published as US patent documents identified as U.S. Pat. Nos. 5,835,022, and 6,670,883)
On the transmission side unit of these systems, Biphase code or Manchester code is used to encode digital data (NRZ format data) for transmission, and the encoded digital data is used to modulate the radio signal that is to be transmitted from an antenna by using a carrier wave in, for example, the UHF band. Therefore, on the reception side of the system, the radio signal is received and de-modulated for decoding the digital data by determining whether a binary level of the de-modulated analog signal is high or low. In this manner, the digital data is transmitted and restored (i.e., being played back).
As shown in FIG. 12A, in the Manchester coding, a data bit of 1 having a logical value of 1 and a data bit of 0 having a logical value of 0 are distinguished by examining a rise (i.e., a change from a low level to a high level) or a fall (i.e., a change from a high level to a low level) in the middle of one bit period. Therefore, for example, the bit1 in the Manchester coding is encoded as the fall signal in the one bit period, and the bit0 is encoded as the rise signal in the one bit period.
Further, as shown in FIG. 12B, in the Biphase coding, the data bit of 0 is coded as a signal that inverts its (high-low) level in the middle of the one bit period, an the bit of 1 is coded as a signal that inverts its high-low level at an end of the one bit period, with an additional inversion of the signal level of each bit that a high-level ending of the previous bit is succeeded by a low-level start of the next bit, and a low-level ending of the previous bit is succeeded by the high-level start of the next bit.
Therefore, in both of the Manchester coding and the Biphase coding, each bit of the digital data is coded as a signal that has at least one binary level change between the high level and the low level in the middle of the bit or at the end of the bit. Further, due to the scheme of coding described above, the clock of the signal can be restored from the digital signal after coding.
The vehicle communication system having the above-described coding scheme may possibly have a distorted wave form in the demodulated analog signal received and demodulated by the receiving side unit relative to the original data due to an external noise or the like. The distorted wave form on the receiving side unit may lead to a mis-determination of the binary signal level in the demodulated analog signal, thereby yielding an erroneous logical value as the result of the decoding of the original digital data.